The death toll in a Florida nightclub mass shooting rose to 50, with at least 53 other people injured, the Orlando mayor said on Sunday.

Police said the gunman, identified as 29-year-old Omar Mateen of Fort Pierce in Florida, had wielded an assault-type rifle and a handgun when he took hostages and opened fire inside the crowded Florida nightclub. Police desribed the shooting as a"mass casualty".

Police said the shooter was dead, four hours after the incident began with a volley of gunfire in the Pulse nightclub in downtown Orlando, triggering a massive deployment of police and emergency vehicles.

Police Chief John Mina also said the shooter had some sort of "suspicious device." He said the suspect exchanged gunfire with an officer working at the club around 2 am, then went back inside and took hostages.

Around 5am, authorities sent in a SWAT team to rescue the hostages, and the suspect then died in a gunfight with those officers. Mina said police have not determined an exact number of casualties, but that "approximately 20" people were dead inside the club.

It was the second shooting incident in the city in just over 24 hours, coming shortly after singer Christina Grimmie was shot dead late Friday by a gunman who stormed a theater where she had just finished a gig. There was no link seen between the night club shooting and the murder of Grimmie.

Witnesses said the shooting erupted at around 2am (6am GMT) amid throbbing music as the crowded club was nearing closing time.

Speaking to Sky News, clubber Ricardo Negron, who was inside when the shooting began, described how the gunman raked the club with bullets.

"People just dropped on the floor. I guess the shooter was shooting at the ceiling because you could see all the glass from the lamps falling," he told the network.

He described hearing "non-stop firing" which probably lasted less than a minute but felt like a lot longer.

"There was a brief pause in the shooting and some of us just got up and ran out the back."

"People have definitely been injured - or worse."

Video posted online showed a large response by police and emergency vehicles outside the nightclub.

'Everyone get out'

Emergency vehicles swarmed the area, with at least one crisis command vehicle at the scene.

"I didn't see any of the actual shooters. I just saw bodies going down and I was ordering a drink at the bar.

"I fell down. I crawled out. People were trying to escape out the back. I just know that when I hit the ground, I was crawling and I hit my elbows and my knees," he told CNN.

He said there was "blood everywhere."

Rosie Feba was at the club with a friend when the shooting broke out.

"She told me someone was shooting. Everyone was getting on the floor," Feba told the Orlando Sentinel. "I told her I didn't think it was real, I thought it was just part of the music, until I saw fire coming out of his gun."

The incident came on the heels of a deadly shooting at the nearby Plaza Live Theater in which a heavily-armed gunman shot and critically wounded singer Grimmie, a former contestant on the popular TV talent show "The Voice."

She was rushed to hospital but died of her wounds.

The theater is located less than four miles from Pulse nightclub.

The Plaza gunman, 27-year-old Kevin James Loibl, then killed himself.

He was found to be armed with armed with two handguns, ammunition and a hunting knife, police said. His motive remains unknown.